In the prior art, a variable transmission ratio apparatus using a strain wave gear mechanism as a differential gear mechanism adds rotation resulting from motor drive to rotation from an input shaft resulting from a steering operation and transmits the resulting rotation from the input shaft to an output shaft. Such a variable transmission ratio apparatus may be of a housing-integrated type in which a housing rotates integrally with an input shaft (refer, for example, to patent document 1 below), or may be of a housing-fixed type in which a housing is fixed to a non-rotating portion of a vehicle (refer, for example, to patent document 2 below).
The strain wave gear mechanism may include, for example, a pair of circular splines, with one of the circular splines having a greater number of teeth than the flexible spline. The difference in the number of teeth causes a rotation difference during rotation of the mated portion. This yields a reduction ratio.
Normally, to make the apparatus fail-safe or when turning off an ignition switch of a vehicle, a variable transmission ratio apparatus using a strain wave gear mechanism includes a lock device for locking a motor shaft connected to a wave generator and a housing to prevent relative rotation. Activation of the lock device restricts free rotation of the motor shaft and maintains a state in which the transmission of torque between the input shaft and the output shaft is enabled.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-9982
Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-175336